Capacitor with electrodes made of an interconnected corrugated carbon-based network

ABSTRACT

Capacitors having electrodes made of interconnected corrugated carbon-based networks (ICCNs) are disclosed. The ICCN electrodes have properties that include high surface area and high electrical conductivity. Moreover, the electrodes are fabricated into an interdigital planar geometry with dimensions that range down to a sub-micron scale. As such, micro-supercapacitors employing ICCN electrodes are fabricated on flexible substrates for realizing flexible electronics and on-chip applications that can be integrated with micro-electromechanical systems (MEMS) technology and complementary metal oxide semiconductor technology in a single chip. In addition, capacitors fabricated of ICCN electrodes that sandwich an ion porous separator realize relatively thin and flexible supercapacitors that provide compact and lightweight yet high density energy storage for scalable applications.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 16/029,930, filed Jul. 9, 2018, which is a continuation of U.S.patent application Ser. No. 15/612,405, filed Jun. 2, 2017, now U.S.Pat. No. 11,004,618, which is a continuation of U.S. patent applicationSer. No. 14/382,463, filed Sep. 2, 2014, now U.S. Pat. No. 9,779,884.

U.S. patent application Ser. No. 14/382,463, filed Sep. 2, 2014, is aUSC 371 national phase filing of PCT/US13/29022, filed Mar. 5, 2013,which claims priority to U.S. Provisional Patent Applications No.61/606,637, filed Mar. 5, 2012, and No. 61/757,077, filed Jan. 25, 2013.

The present application is related to U.S. patent application Ser. No.13/725,073, filed Dec. 21, 2012, and International Patent ApplicationNo. PCT/US12/71407, filed Dec. 21, 2012, both of which claim priority toU.S. Provisional Patent Application No. 61/578,431, filed Dec. 21, 2011.

All of the applications listed above are hereby incorporated herein byreference in their entireties.

This research was supported in part by the Ministry of Higher Educationof Egypt through a graduate research fellowship—the Missions Program.

FIELD OF THE DISCLOSURE

The present disclosure provides an interconnected corrugatedcarbon-based network (ICCN) and an inexpensive process for making,patterning, and tuning the electrical, physical and electrochemicalproperties of the ICCN.

BACKGROUND

Batteries and electrochemical capacitors (ECs) stand at opposite ends ofthe spectrum in terms of their power and energy densities. Batteriesstore energy through electrochemical reactions and can exhibit highenergy densities (on the order of 20 to 150 Wh/kg), whereas ECs, whichstore charge in electrochemical double layers (EDLs), can only achievevalues of 4 to 5 Wh/kg. However, because ion flow is faster than redoxreactions, ECs can deliver much higher power densities. ECs are alsogenerally maintenance free and display a longer shelf and cycle life, sothey are often favored in many electronic applications.

An EC that combines the power performance of capacitors with the highenergy density of batteries would represent a major advance in energystorage technology, but this requires an electrode with higher and moreaccessible surface area than that of conventional EC electrodes whilemaintaining high conductivity. Carbon-based materials are attractive inthis regard because of their mechanical and electrical properties aswell as exceptionally high surface area. Recently, the intrinsiccapacitance of single layer graphene was reported to be ˜21 μF/cm²; thisvalue now sets the upper limit for EDL capacitance for all carbon-basedmaterials. Thus, ECs based on carbon-based materials could, inprinciple, achieve an EDL capacitance as high as ˜550 F/g if theirentire surface area could be used.

Currently, carbon-based materials derived from graphite oxide (GO) canbe manufactured on the ton scale at low cost, making them potentiallycost effective materials for charge storage devices. Although thesecarbon-based materials have shown excellent power density and life-cyclestability, their specific capacitance (130 F/g in aqueous potassiumhydroxide and 99 F/g in an organic electrolyte) still falls far belowthe theoretical value of ˜550 F/g calculated for a single layer ofcarbon. A variety of other carbon-based materials derived from GO havealso been used, yet the values of specific capacitance, energy density,and power density have remained lower than expected. These effects areoften attributed to the restacking of carbon sheets during processing asa result of the strong sheet-to-sheet van der Waals interactions. Thisreduction in the specific surface area of single layer carbon accountsfor the overall low capacitance. In addition, these ECs exhibitedrelatively low charge/discharge rates, which precludes their use forhigh power applications. Recently, EC devices composed of curvedgraphene, activated graphene, and solvated graphene have demonstratedenhanced performance in terms of energy density. However, furtherimprovements in energy density are needed that do not sacrifice highpower density. In particular, the production of mechanically robustcarbon-based electrodes with large thicknesses (˜10 μm or higher) andhigh surface-to-volume ratio in a binder free process would result inhigh power and high energy density ECs.

In the pursuit of producing high quality bulk carbon-based devices suchas ECs and organic sensors, a variety of syntheses now incorporategraphite oxide (GO) as a precursor for the generation of large scalecarbon-based materials. Inexpensive methods for producing largequantities of GO from the oxidation of graphitic powders are nowavailable. In addition, the water dispersibility of GO combined withinexpensive production methods make GO an ideal starting material forproducing carbon-based devices. In particular, GO has water dispersibleproperties. Unfortunately, the same oxygen species that give GO itswater dispersible properties also create defects in its electronicstructure, and as a result, GO is an electrically insulating material.Therefore, the development of device grade carbon-based films withsuperior electronic properties requires the removal of these oxygenspecies, re-establishment of a conjugated carbon network, as well as amethod for controllably patterning carbon-based device features.

Methods for reducing graphite oxide have included chemical reduction viahydrazine, hydrazine derivatives, or other reducing agents, hightemperature annealing under chemical reducing gases and/or inertatmospheres, solvothermal reduction, a combination of chemical andthermal reduction methods, flash reduction, and most recently, laserreduction of GO. Although several of these methods have demonstratedrelatively high quality graphite oxide reduction, many have been limitedby expensive equipment, high annealing temperatures and nitrogenimpurities in the final product. As a result, of these difficulties, acombination of properties that includes high surface area and highelectrical conductivity in an expanded interconnected carbon network hasremained elusive. In addition, large scale film patterning via anall-encompassing step for both GO reduction and patterning has provendifficult and has typically been dependent on photo-masks to provide themost basic of patterns. Therefore, what is needed is an inexpensiveprocess for making and patterning an interconnected corrugatedcarbon-based network (ICCN) having a high surface area with highlytunable electrical conductivity and electrochemical properties.

SUMMARY

The present disclosure provides a capacitor having at least oneelectrode made up of an interconnected corrugated carbon-based network(ICCN). The ICCN produced has a combination of properties that includeshigh surface area and high electrical conductivity in an expandednetwork of interconnected carbon layers.

In one embodiment, each of the expanded and interconnected carbon layersis made up of at least one corrugated carbon sheet that is one atomthick. In another embodiment, each of the expanded and interconnectedcarbon layers is made up of a plurality of corrugated carbon sheets thatare each one atom thick. The interconnected corrugated carbon-basednetwork is characterized by a high surface area with highly tunableelectrical conductivity and electrochemical properties.

In one embodiment, a method produces a capacitor having electrodes madeof a patterned ICCN. In that particular embodiment, an initial stepreceives a substrate having a carbon-based oxide film. Once thesubstrate is received, a next step involves generating a light beamhaving a power density sufficient to reduce portions of the carbon-basedoxide film to an ICCN. Another step involves directing the light beamacross the carbon-based oxide film in a predetermined pattern via acomputerized control system while adjusting the power density of thelight beam via the computerized control system according topredetermined power density data associated with the predeterminedpattern.

In one embodiment, the substrate is a disc-shaped, digital versatiledisc (DVD) sized thin plastic sheet removably adhered to a DVD sizedplate that includes a DVD centering hole. The DVD sized plate carryingthe disc-shaped substrate is loadable into a direct-to-disc labelingenabled optical disc drive. A software program executed by thecomputerized control system reads data that defines the predeterminedpattern. The computerized control system directs a laser beam generatedby the optical disc drive onto the disc-shaped substrate, therebyreducing portions of the carbon-based oxide film to an electricallyconductive ICCN that matches shapes, dimensions, and conductance levelsdictated by the data of the predetermined pattern.

Those skilled in the art will appreciate the scope of the disclosure andrealize additional aspects thereof after reading the following detaileddescription in association with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of thisspecification illustrate several aspects of the disclosure, and togetherwith the description serve to explain the principles of the disclosure.

FIG. 1 depicts the label side of a prior art direct-to-disc labelingtype CD/DVD disc.

FIG. 2 is a schematic of a prior art direct-to-disc labeling typeoptical disc drive.

FIG. 3 is a process diagram for an exemplary process for providinggraphite oxide (GO) films on a substrate.

FIG. 4 is a process diagram for laser scribing an interconnectedcorrugated carbon-based network (ICCN) and then fabricating electricalcomponents from the ICCN.

FIG. 5 is a line drawing of a sample of the ICCN of the presentembodiments.

FIG. 6A is an artwork image of a man's head covered with circuits.

FIG. 6B is a photograph of a GO film after the artwork image of FIG. 6Ais directly patterned on the GO film using the laser scribing techniqueof the present disclosure.

FIG. 7 is a graph that provides a comparison between changes inelectrical conductivity by reducing the GO film of FIG. 6B by usingvarious grayscale levels to laser scribe the artwork of FIG. 6A toproduce the patterned GO film of FIG. 6B.

FIG. 8A is a scanning electron microscope (SEM) image that illustratesan infrared laser's effect on GO film prior to laser treatment on theright side of the image in contrast to an aligned ICCN on the left sideof the image.

FIG. 8B is an SEM image showing that an ICCN has a thickness that isapproximately 10 times larger in comparison to that of untreated GOfilm.

FIG. 8C is an SEM image showing a cross-sectional view of a single laserconverted ICCN.

FIG. 8D is an SEM image showing a greater magnification of a selectedarea within the ICCN in FIG. 8C.

FIG. 9 compares a powder X-ray diffraction (XRD) pattern of the ICCNwith both graphite and graphite oxide diffraction patterns.

FIG. 10 is a plot of log₁₀ of peak current versus log₁₀ of an appliedvoltammetric scan rate.

FIGS. 11A-11E are graphs related to Raman spectroscopy analysis.

FIG. 12A is a graph depicting an electrical resistance change of aflexible ICCN electrode as a function of a bending radius.

FIG. 12B is a graph depicting an electrical resistance change of aflexible ICCN electrode as a function of bending cycles.

FIG. 13A is a cyclic voltammetry graph comparing a GO electrochemicalcapacitor (EC) with an ICCN EC.

FIG. 13B is a graph depicting galvanostatic charge/discharge (CC) curvesof an ICCN EC measured at a high current density of 10A/g_(ICCN/electrode).

FIG. 13C is a graph of volumetric stack capacitance of an ICCN EC thatis calculated from the CC curves at different charge/discharge currentdensities.

FIG. 13D is a graph of ICCN EC cyclic stability versus CC cycles.

FIG. 13E is a graph of a complex plane plot of the impedance of an ICCNEC, with a magnification for the high-frequency region in a graph inset.

FIG. 13F is a graph of impedance phase angle versus frequency for anICCN EC and a commercial activated carbon EC.

FIG. 14A is a structural diagram of an assembled ICCN EC.

FIG. 14B is a graph of stack capacitance as a function of currentdensity.

FIG. 14C is a graph of capacitance retention for the ICCN EC over a 4month period.

FIG. 14D is a graph of cyclic voltammetry (CV) performance of the ICCNEC when tested under different bending conditions.

FIG. 14E is a graph of galvanostatic charge/discharge curves for fourtandem ICCN ECs connected in series.

FIG. 14F is a graph of galvanostatic charge/discharge curves for fourICCN ECs in a series and parallel combination.

FIG. 15 is a graph of galvanostatic charge/discharge curves of thedevice when operated at an ultrahigh current density of 250A/g_(ICCN/electrode).

FIG. 16 is a Ragone plot comparing the performance of ICCN ECs withdifferent energy storage devices designed for high powermicroelectronics.

FIG. 17A is a structural diagram showing a set of interdigitatedelectrodes made of ICCNs with dimensions of 6 mm×6 mm, spaced at aroundabout 500 μm, that are directly patterned onto a thin film of GO.

FIG. 17B is a structural diagram showing the set of interdigitatedelectrodes transferred onto another type of substrate.

FIG. 18A shows an exploded view of a micro-supercapacitor made up of aplurality of expanded and interconnected carbon layers that areelectrically conductive.

FIG. 18B shows the micro-supercapacitor of FIG. 18A after assembly.

FIG. 19A depicts a micro-supercapacitor configuration having a firstelectrode with two extending electrode digits that are interdigitatedwith two extending electrode digits of a second electrode.

FIG. 19B depicts a micro-supercapacitor configuration having a firstelectrode with four extending electrode digits that are interdigitatedwith four extending electrode digits of a second electrode.

FIG. 19C depicts a micro-supercapacitor configuration having a firstelectrode with eight extending electrode digits that are interdigitatedwith eight extending electrode digits of a second electrode.

FIG. 20 is a table listing dimensions for the micro-supercapacitors ofFIGS. 19A-19C.

FIGS. 21A-21E depict the fabrication of ICCN micro-supercapacitors.

FIG. 22A depicts ICCN micro-devices with 4, 8, and 16 interdigitatedelectrodes.

FIG. 22B depicts an ICCN micro-device with 16 interdigitated fingerswith 150-μm spacings.

FIG. 22C is a tilted view (45°) SEM image that shows the directreduction and expansion of the GO film after exposure to the laser beam.

FIGS. 22D and 22E show I-V curves of GO and an ICCN, respectively.

FIG. 22F is a graphical comparison of electrical conductivity values forGO and an ICCN.

FIGS. 23A-23I are graphs depicting electrochemical performance of ICCNmicro-supercapacitors in PVA-H₂SO₄ gelled electrolyte.

FIGS. 24A-24F are graphs depicting the behavior of ICCNmicro-supercapacitors under mechanical stress in series and parallelconfigurations.

FIGS. 25A-25E are images depicting the fabrication of ICCNmicro-supercapacitors on a chip along with graphs showing thecharacteristics of the micro-supercapacitors.

FIGS. 26A-26B are graphs depicting self discharge rates for ICCNmicro-supercapacitors.

FIG. 27 is a Ragone plot of energy and power densities of ICCNmicro-supercapacitors compared with commercially available energystorage systems.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the disclosure andillustrate the best mode of practicing the disclosure. Upon reading thefollowing description in light of the accompanying drawings, thoseskilled in the art will understand the concepts of the disclosure andwill recognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

The present disclosure provides an inexpensive process for making andpatterning an ICCN having stringent requirements for a high surface areawith highly tunable electrical conductivity and electrochemicalproperties. The embodiments described herein not only meet thesestringent requirements, but provide direct control over the conductivityand patterning of an ICCN while creating flexible electronic devices ina single step process. Moreover, the production of the ICCN does notrequire reducing agents, or expensive equipment. The simple directfabrication of an ICCN on flexible substrates therefore simplifies thedevelopment of lightweight electrical energy storage devices. The ICCNcan be synthesized on various substrates, such as plastic, metal, andglass. Herein an electrochemical capacitor (EC), and in particular amicro-supercapacitor, is disclosed.

In at least one embodiment, the ICCNs are conducting films producedusing a common and inexpensive infrared laser that fits inside a compactdisc/digital versatile disc (CD/DVD) optical drive unit that provides adirect-to-disc label writing function. LightScribe (Registered Trademarkof Hewlett Packard Corporation) and LabelFlash (Registered Trademark ofYamaha Corporation) are exemplary direct-to-disc labeling technologiesthat pattern text and graphics onto the surface of a CD/DVD disc.LightScribe DVD drives are commercially available for around $20 and theLightScribing process is controlled using a standard desktop computer.

FIG. 1 depicts the label side of a standard direct-to-disc labeling typeCD/DVD disc 10 that includes a label area 12 and a clamping area 14 thatsurrounds a centering hole 16. A dye film 18 covers the label area 12and is sensitive to laser energy that is typically directed onto thelabel area 12 to produce a permanent visible image that may comprisegraphics 20 and text 22. A position tracking indicia 24 is usable by anoptical disc drive (not shown) to accurately locate an absolute angularposition of the CD/DVD disc 10 within the optical disc drive so that thegraphics 20 and/or text 22 can be re-written to provide increasedcontrast. Moreover, the position tracking indicia 24 is usable by theoptical disc drive to allow additional graphics and/or text to bewritten without undesirably overwriting the graphics 20 and/or text 22.

FIG. 2 is a schematic of a prior art direct-to-disc labeling typeoptical disc drive system 26. In this exemplary case, the CD/DVD disc 10is depicted in cross-section and loaded onto a spindle assembly 28 thatis driven by a CD/DVD spindle motor 30. The label area 12 is shownfacing a laser assembly 32 that includes a label writer laser (LWL) 34,a lens 36, and a focus actuator 38. The LWL 34 is typically a laserdiode. Exemplary specifications for the LWL 34 includes a maximum pulseoptical power of 350 mW at 780 nm emission and a maximum pulse outputpower of 300 mW at 660 nm emission. A laser beam 40 emitted by the LWL34 is focused by the lens 36 that is alternately translated towards andaway from the LWL 34 by the focus actuator 38 in order to maintain focusof the laser beam 40 onto the label area 12 of the CD/DVD disc 10. Thelaser beam 40 is typically focused to a diameter that ranges from around0.7 μm to around 1 μm.

The laser assembly 32 is responsive to a control system 42 that providescontrol signals 44 through an optical drive interface (ODI) 46. Thecontrol system 42 further includes a central processor unit (CPU) 48 anda memory 50. Label image data (LID) having information needed to realizea permanent image to be written onto the label area 12 of the CD/DVDdisc 10 is processed by the CPU 48, which in turn provides an LID streamsignal 52 that pulses the LWL 34 on and off to heat the dye film 18 torealize the image defined by the LID.

The CPU 48 also processes the LID through the ODI 46 to provide aposition control signal 54 to a radial actuator 56 that translates thelaser assembly 32 in relation to the label area 12 in response toposition information contained in the LID. In some versions of thepresent embodiments, the optical disc drive system 26 monitors the focusof the laser beam 40 with an optical receiver (not shown), so that theODI 46 can generate a focus control signal 58 for the focus actuator 38.The ODI 46 also provides a motor control signal 60 for the CD/DVDspindle motor 30 that maintains an appropriate rotation speed of theCD/DVD disc 10 while a label writing process is ongoing.

In some versions of the optical disc drive system 26 the LWL 34 is usedexclusively for label writing directly to the label area 12 of theCD/DVD disc and a separate laser diode (not shown) is used to writeand/or read data to/from a data side 62 of the CD/DVD disc 10. In otherversions of the optical disc drive system 26, the LWL 34 is used forlabel writing and data reading and/or writing. When the LWL 34 is usedfor data reading and/or writing, the CD/DVD disc 10 is flipped over toexpose the data side 62 of the CD/DVD disc 10 to the laser beam 40. Inversions wherein the LWL 34 is also used as a data read/write laser, thelaser assembly 32 includes optical pick-up components (not shown) suchas a beam splitter and at least one optical receiver. The output powerof the LWL 34 is typically around 3 mW during data read operations.

In order to use the optical disc drive system 26 to realize aninexpensive process for making and patterning an ICCN having a highsurface area with highly tunable electrical conductivity andelectrochemical properties, a carbon-based film is substituted for thedye film 18 (FIG. 1). In one embodiment, graphite oxide (GO) issynthesized from high purity graphite powder using a modified Hummer'smethod. Dispersions of GO in water (3.7 mg/mL) are then used to make GOfilms on various substrates. Exemplary substrates include but are notlimited to polyethylene terephthalate (PET), nitrocellulose membrane(with 0.4 μm pore size), aluminum foil, carbonized aluminum, copperfoil, and regular copier paper.

Referring to FIG. 3, a process 100 begins with providing graphite powder64. The graphite powder 64 undergoes an oxidation reaction using themodified Hummer's method to become GO 66 (step 102). However, it is tobe understood that other oxidation methods for producing GO areavailable and such methods are within the scope of the presentdisclosure. An exfoliation procedure produces exfoliated GO 68 (step104). The exfoliation procedure may be accomplished via ultrasonication.It is to be understood that the exfoliated GO 68 results from a partialexfoliation and not a complete exfoliation to a single layer of GO. Thepartial exfoliation is used to create a high accessible surface areathat enables a fast redox response which enables a fast sensor response.Additionally, the partial exfoliation of GO 68 provides the high surfacearea for growing metal nanoparticles that could then be used incatalysis. A substrate 70 carries a GO film 72 that is produced by adeposition procedure that deposits the exfoliated GO 68 onto thesubstrate 70 (step 106). In at least some embodiments, a GO film 72 ismade by either drop-casting or vacuum filtering GO dispersions onto thesubstrate 70 that is the size of a CD/DVD disc. The GO film 72 istypically allowed to dry for 24 hours under ambient conditions. However,controlling conditions to expose the GO film 72 to a relatively lowerhumidity and relatively higher temperature will dry the GO film 72relatively quickly. The term GO herein refers to graphite oxide.

Referring to FIG. 4, individual ones of the GO film(s) 72 are thenaffixed to a substrate carrier 74, which has dimensions similar to theCD/DVD disc 10 (FIG. 1)(step 200). The substrate carrier 74 carrying thesubstrate 70 with the GO film 72 is loaded into the optical disc drivesystem 26 (FIG. 2) such that the GO film 72 faces the LWL 34 for lasertreatment (step 202). In this way, the present embodiments use the GOfilm 72 in place of the dye film 18 (FIG. 1). It is to be understoodthat the substrate carrier 74 can be a rigid or semi-rigid disc ontowhich the GO film 72 can be fabricated directly. In that case, thesubstrate carrier 74 replaces the function of the substrate 70.

Images 76 for realizing electrical components 78 are patterned inconcentric circles, moving outward from the center of the substratecarrier 74 (step 204). The laser irradiation process results in theremoval of oxygen species and the reestablishment of sp² carbons. Thiscauses a change in the conductivity of the GO film 72 with a typicalresistance of >20 MΩ/sq to become a relatively highly conductingplurality of expanded and interconnected carbon layers that make up anICCN 80. The number of times the GO film 72 is laser treated results ina significant and controllable change in the conductivity of the ICCN80. The ICCN 80 has a combination of properties that includes highsurface area and high electrical conductivity in an expandedinterconnected network of carbon layers. In one embodiment, theplurality of expanded and interconnected carbon layers has a surfacearea of greater than around about 1400 m²/g. In another embodiment, theplurality of expanded and interconnected carbon layers has a surfacearea of greater than around about 1500 m²/g. In yet another embodiment,the surface area is around about 1520 m²/g. In one embodiment, theplurality of expanded and interconnected carbon layers yields anelectrical conductivity that is greater than around about 1500 S/m. Inanother embodiment, the plurality of expanded and interconnected carbonlayers yields an electrical conductivity that is greater than aroundabout 1600 S/m. In yet another embodiment, the plurality of expanded andinterconnected carbon layers yields an electrical conductivity of aroundabout 1650 S/m. In still another embodiment, the plurality of expandedand interconnected carbon layers yields an electrical conductivity thatis greater than around about 1700 S/m. In yet one more embodiment, theplurality of expanded and interconnected carbon layers yields anelectrical conductivity of around about 1738 S/m. Moreover, in oneembodiment, the plurality of expanded and interconnected carbon layersyields an electrical conductivity that is greater than around about 1700S/m and a surface area that is greater than around about 1500 m²/g. Inanother embodiment, the plurality of expanded and interconnected carbonlayers yields an electrical conductivity of around about 1650 S/m and asurface area of around about 1520 m²/g.

The electrical components 78 comprising electrodes 82 used in thefabrication of an electrochemical capacitor (EC) 84 are laser irradiated6 times before reaching the relatively high conductivity of around about1738 S/m. An exemplary laser irradiation process takes around about 20minutes per cycle. However, it is to be understood that faster laserirradiation rates are possible depending on the power of the laser lightemitted from the LWL 34 combined with an increased positioning rate ofthe substrate carrier. Moreover, other imaging techniques that employphotomasks and flashlamps may provide even faster fabrication of theelectrical components 78. Afterwards, the substrate 70 carrying the ICCN80 and any remaining GO film 72 is removed from the substrate carrier 74(step 206). Next, the ICCN 80 is fabricated into the electricalcomponents 78 that make up the EC 84 (step 208). In this exemplary case,portions of the ICCN 80 on the substrate 70 are cut into rectangularsections to make the electrical components 78, which include theelectrodes 82 formed from the ICCN 80. A separator/electrolyte 86 issandwiched between the electrodes 82 to form the EC 84.

The ICCN 80 possesses a very low oxygen content of only around about3.5%, which contributes to a relatively very high charging rate. Inother embodiments, the oxygen content of the expanded and interconnectedcarbon layers ranges from around about 1% to around about 5%. FIG. 5 isa line drawing of a sample of the ICCN 80, which is made up of theplurality of expanded and interconnected carbon layers that includecorrugated carbon layers such as a single corrugated carbon sheet 88. Inone embodiment, each of the expanded and interconnected carbon layerscomprises at least one corrugated carbon sheet that is one atom thick.In another embodiment, each of the expanded and interconnected carbonlayers comprises a plurality of corrugated carbon sheets 88. Thethickness of the ICCN 80, as measured from cross-sectional scanningelectron microscopy (SEM) and profilometry, was found to be around about7.6 μm. In one embodiment, a range of thicknesses of the plurality ofexpanded and interconnected carbon layers making up the ICCN 80 is fromaround about 7 μm to 8 μm.

As an illustration of the diversity in image patterning that ispossible, a complex image formed by the direct laser reduction of GO isshown in FIGS. 6A and 6B. FIG. 6A is an artwork image of a man's headcovered with circuits. FIG. 6B is a photograph of a GO film after theartwork image of FIG. 6A is directly patterned on the GO film using thelaser scribing technique of the present disclosure. Essentially, anypart of the GO film that comes in direct contact with the 780 nminfrared laser is effectively reduced to an ICCN, with the amount ofreduction being controlled by the laser intensity; a factor that isdetermined by power density of the laser beam impinging on the GO film.The resulting image of FIG. 6B is an effective print of the originalimage of FIG. 6A. However, in this case the image of FIG. 6B is made upof various reductions of the GO film. As expected, the darkest blackareas indicate exposure to the strongest laser intensities, while thelighter gray areas are only partially reduced. Since different grayscalelevels directly correlate with the laser's intensity, it is possible totune the electrical properties of the generated ICCN over five to sevenorders of magnitude in sheet resistance (Ω/sq) by simply changing thegrayscale level used during the patterning process. As illustrated inFIG. 7, there is a clear relationship between sheet resistance,grayscale level and the number of times the GO film is laser irradiated.Control over conductivity from a completely insulating GO film, with atypical sheet resistance value of >20 MΩ/sq, to a conducting ICCN thatregisters a sheet resistance value of approximately 80 Ω/sq, whichtranslates to a conductivity of around about 1650 S/m, is possible. Thismethod is sensitive enough to differentiate between similar grayscalelevels as shown in the graph of FIG. 7, where the sheet resistancevaries significantly with only a small variation in grayscale level. Inaddition, the number of times a GO film is laser treated results in asignificant and controllable change in sheet resistance. Each additionallaser treatment lowers the sheet resistance as seen in FIG. 7, where afilm is laser irradiated once (black squares), twice (circles) and threetimes (triangles) with respect to the grayscale level. Therefore, thefilm's sheet resistance is tunable both by controlling the grayscalelevel used and the number of times the film is reduced by the laser, aproperty that has so far been difficult to control through othermethods.

Scanning electron microscope (SEM) techniques are usable to understandthe effects a low energy infrared laser has on the structural propertiesof GO film by comparing the morphological differences between an ICCNand untreated graphite oxide GO film. FIG. 8A is an SEM image thatillustrates the infrared laser's effect on GO film prior to lasertreatment on the right side of the image in contrast to an aligned ICCNon the left side of the image that occurs after being reduced with theinfrared laser. The image not only gives a clear definition between theICCN and untreated GO regions, but also demonstrates the level ofprecision possible when using this method as a means to pattern andreduce GO. The regions of ICCN, which result from the laser treatment,can be further analyzed through cross-sectional SEM.

FIG. 8B is an SEM image showing a cross-sectional view of a freestanding film of laser treated and untreated GO film, which shows asignificant difference between GO film thicknesses. As indicated by thewhite brackets in FIG. 8B, an ICCN increases in thickness byapproximately 10 times in comparison to that of untreated GO film.Moreover, a range of thicknesses of the plurality of expanded andinterconnected carbon layers is from around about 7 μm to around 8 μm.In one embodiment, an average thickness of the plurality of expanded andinterconnected carbon layers is around about 7.6 μm. The increasedthickness stems from rapid degassing of gases generated and releasedduring laser treatment, similar to thermal shock, which effectivelycauses the reduced GO to expand and exfoliate as these gases rapidlypass through the GO film. FIG. 8C is an SEM image showing across-sectional view of a single ICCN, which shows an expanded structurethat is a characteristic of the ICCN of the present disclosure.

FIG. 8D is an SEM image showing a greater magnification of a selectedarea within the ICCN in FIG. 8C. The SEM image of FIG. 8D allows thethickness of the plurality of expanded and interconnected carbon layersto be calculated to be between around about 5-10 nm. However, the numberof carbon layers in the plurality of expanded and interconnected carbonlayers making up the ICCN is greater than around about 100. In anotherembodiment the number of carbon layers in the plurality of expanded andinterconnected carbon layers is greater than around about 1000. In yetanother embodiment the number of carbon layers in the plurality ofexpanded and interconnected carbon layers is greater than around about10,000. In still another embodiment, the number of carbon layers in theplurality of expanded and interconnected carbon layers is greater thanaround about 100,000. The SEM analysis shows that although an infraredlaser emission is only marginally absorbed by GO, enough power and focus(i.e., power density) can cause sufficient thermal energy to efficientlyreduce, deoxygenate, expand, and exfoliate the GO film. Moreover, thesurface area of the ICCN is greater than around about 1500 m²/g.

Since each of the carbon layers has a theoretical surface area of aroundabout 2630 m²/g, a surface greater than around about 1500 m²/g indicatesthat almost all surfaces of the carbon layers are accessible. The ICCNhas an electrical conductivity that is greater than around about 17S/cm. The ICCN forms when some wavelength of light hits the surface ofthe GO, and is then absorbed to practically immediately convert to heat,which liberates carbon dioxide (CO₂). Exemplary light sources includebut are not limited to a 780 nm laser, a green laser, and a flash lamp.The light beam emission of the light sources may range from nearinfrared to ultraviolet wavelengths. The typical carbon content of theICCN is greater than around about 97% with less than around about 3%oxygen remaining. Some samples of the ICCN are greater than around about99% carbon even though the laser reduction process is conducted in theair.

FIG. 9 compares a powder X-ray diffraction (XRD) pattern of thecorrugated carbon-based network with both graphite and graphite oxidediffraction patterns. A typical XRD pattern for graphite, shown in FIG.9 trace A, displays the characteristic peak of 2θ=27.8° with a d-spacingof 3.20 Å. An XRD pattern (FIG. 9, trace B) for GO, on the other hand,exhibits a single peak of 2θ=10.76°, which corresponds to an interlayerd-spacing of 8.22 Å. The increased d-spacing in GO is due to the oxygencontaining functional groups in graphite oxide sheets, which tend totrap water molecules between the basal planes, causing the sheets toexpand and separate. The XRD pattern of the corrugated carbon-basednetwork (FIG. 9, trace C) shows the presence of both GO (10.76° 2θ) anda broad graphitic peak at 25.97° 2θ associated with a d-spacing of 3.43Å. The GO presence in the corrugated carbon-based network is expectedsince the laser has a desirable penetration depth, which results in thereduction of only the top portion of the film with the bottom layerbeing unaffected by the laser. The small presence of GO is moreprominent in thicker films, but begins to diminish in thinner films. Inaddition, one can also observe a partially obstructed peak at 26.66° 2θ,which shows a similar intensity to the broad 25.97° 2θ peak. Both ofthese peaks are considered graphitic peaks, which are associated to twodifferent lattice spacing between basal planes.

It has been previously shown that the immobilization of carbon nanotubes(CNTs) on glassy carbon electrodes will result in a thin CNT film, whichdirectly affects the voltammetric behavior of the CNT modifiedelectrodes. In a ferro/ferrocyanide redox couple, the voltammetriccurrent measured at the CNT modified electrode will likely have twotypes of contributions. The thin layer effect is a significantcontributor to the voltammetric current. The thin layer effect stemsfrom the oxidation of ferrocyanide ions, which are trapped between thenanotubes. The other contribution results from the semi-infinitediffusion of ferrocyanide towards the planar electrode surface.Unfortunately, the mechanistic information is not easily de-convolutedand requires knowledge of the film thickness.

In contrast, no thin layer effect is observed in association with theinterconnected corrugated carbon-based network of the presentdisclosure. FIG. 10 is a plot of log₁₀ of peak current versus log₁₀ ofan applied voltammetric scan rate. In this case, no thin layer effect isobserved since the plot has a consistent slope of 0.53 and is linear.The slope of 0.53 is relatively close to theoretical values calculatedusing a semi-infinite diffusion model governed by the Randles-Sevcikequation:

$i_{p} = {{0.3}443AC_{o}^{*}\sqrt{\frac{D_{o}{v\left( {n\; F} \right)}^{3}}{RT}}}$

Raman spectroscopy is used to characterize and compare the structuralchanges induced by laser treating GO film. FIGS. 11A-11E are graphsrelated to Raman spectroscopic analysis. As can be seen in FIG. 11A,characteristic D, G, 2D and S3 peaks are observed in both GO and theICCN. The presence of the D band in both spectra suggests that carbonsp³ centers still exist after reduction. Interestingly, the spectrum ofthe ICCN shows a slight increase in the D band peak at around about 1350cm⁻¹. This unexpected increase is due to a larger presence of structuraledge defects and indicates an overall increase in the amount of smallergraphite domains. The result is consistent with SEM analysis, where thegeneration of exfoliated accordion-like graphitic regions (FIG. 5)caused by the laser treatment creates a large number of edges. Howeverthe D band also shows a significant overall peak narrowing, suggesting adecrease in these types of defects in the ICCN. The G band experiences anarrowing and a decrease in peak intensity as well as a peak shift fromaround about 1585 to 1579 cm⁻¹. These results are consistent with there-establishment of sp² carbons and a decrease in structural defectswithin the basal planes. The overall changes in the G band indicate atransition from an amorphous carbon state to a more crystalline carbonstate. In addition, a prominent and shifted 2D peak from around about2730 to around about 2688 cm⁻¹ is seen after GO is treated with theinfrared laser, indicating a considerable reduction of the GO film andstrongly points to the presence of a few-layer interconnected graphitestructure. In one embodiment, the 2D Raman peak for the ICCN shifts fromaround about 2700 cm⁻¹ to around about 2600 cm⁻¹ after the ICCN isreduced from a carbon-based oxide. Moreover, as a result of latticedisorder, the combination of D-G generates an S3 second order peak,which appears at around about 2927 cm⁻¹ and, as expected, diminisheswith decreasing disorder after infrared laser treatment. In someembodiments, the plurality of expanded and interconnected carbon layershas a range of Raman spectroscopy S3 second order peak that ranges fromaround about 2920 cm⁻¹ to around about 2930 cm⁻¹. The Raman analysisdemonstrates the effectiveness of treating GO with an infrared laser asa means to effectively and controllably produce the ICCN.

X-ray photoelectron spectroscopy (XPS) was employed to correlate theeffects of laser irradiation on the oxygen functionalities and tomonitor the structural changes on the GO film. Comparing the carbon tooxygen (C/O) ratios between GO and the ICCN provides an effectivemeasurement of the extent of reduction achieved using a simple lowenergy infrared laser. FIG. 11B illustrates the significant disparitybetween the C/O ratios before and after laser treatment of the GO films.Prior to laser reduction, typical GO films have a C/O ratio ofapproximately 2.6:1, corresponding to a carbon/oxygen content of aroundabout 72% and 38%. In one exemplary embodiment, the ICCN has an enhancedcarbon content of around about 96.5% and a diminished oxygen content ofaround about 3.5%, giving an overall C/O ratio of 27.8:1. In yet anotherexemplary embodiment, a laser reduction of GO results in a C/O ratio of333:1, which is around about 0.3% oxygen content. This relatively lowoxygen content was measured using photoelectron spectroscopy (XPS). Inother embodiments, the plurality of expanded and interconnected carbonlayers has a C/O ratio that ranges from around about 333:1 to aroundabout 25:1. Since the laser reduction process takes place under ambientconditions, it is postulated that some of the oxygen present in the ICCNfilm is a result of the film having a static interaction with oxygenfound in the environment.

FIG. 11C shows that the C1s XPS spectrum of GO displays two broad peaks,which can be resolved into three different carbon componentscorresponding to the functional groups typically found on the GOsurface, in addition to a small π to π* peak at 290.4 eV. Thesefunctional groups include carboxyl, sp³ carbons in the form of epoxideand hydroxyl, and sp² carbons, which are associated with the followingbinding energies: approximately 288.1, 286.8 and 284.6 eV, respectively.

FIG. 11D shows expected results, in that the large degree of oxidationin GO results in various oxygen components in the GO C1s XPS spectrum.These results are in contrast to the ICCN, which shows a significantdecrease in oxygen containing functional groups and an overall increasein the C—C sp² carbon peak. This points to an efficient deoxygenatingprocess as well as the re-establishment of C═C bonds in the ICCN. Theseresults are consistent with the Raman analysis. Thus, an infrared lasersuch as the LWL 34 (FIG. 2) is powerful enough to remove a majority ofthe oxygen functional groups, as is evident in the XPS spectrum of theICCN, which only shows a small disorder peak and a peak at 287.6 eV. Thelatter corresponds to the presence of sp³ type carbons suggesting that asmall amount of carboxyl groups remain in the final product. Inaddition, the presence of a π to π* satellite peak at ˜290.7 eVindicates that delocalized π conjugation is significantly stronger inthe ICCN as this peak is miniscule in the GO XPS spectrum. Theappearance of the delocalized π peak is a clear indication thatconjugation in the GO film is restored during the laser reductionprocess and adds support that an sp² carbon network has beenre-established. The decreased intensity of the oxygen containingfunctional groups, the dominating C═C bond peak and the presence of thedelocalized π conjugation all indicate that a low energy infrared laseris an effective tool in the generation of the ICCN.

FIG. 11E depicts UV-visible light absorbance spectra of GO shown inblack. The inset shows a magnified view of the boxed area showing theabsorbance of GO with respect to a 780 nm infrared laser in the 650 to850 nm region.

Having established that an ICCN has an effective π conjugation, it ispossible to construct devices to make use of the conducting material. Inthis regard, at least one embodiment of the present disclosure providesthe production of ICCN ECs through a simple all-solid-state approachthat avoids the restacking of carbon sheets such as the corrugatedcarbon sheet 88 (FIG. 5). Irradiation of the GO film 72 (FIG. 3) with aninfrared laser such as the LWL 34 (FIG. 2) inside the inexpensivecommercially available direct-to-disc labeling type optical disc drivesystem 26 (FIG. 2) which, as discussed above, reduces the GO film 72 toan ICCN, as indicated by the change in film color from golden brown toblack. Analysis of cross sections of the film with scanning electronmicroscopy showed that the initially stacked GO sheets were convertedinto partially-exfoliated carbon sheets through laser irradiation (FIG.3). The resulting ICCN showed excellent conductivity (around about 1738S/m) as opposed to 10 to 100 S/m for activated carbons, the currentstate-of-the-art material used in commercial devices.

In addition, FIGS. 12A and 12B show that the ICCN made up of corrugatedcarbon sheets shows excellent mechanical flexibility with only aroundabout 1% change in the electrical resistance of the film after 1000bending cycles. Thus, ICCNs can be directly used as EC electrodeswithout the need for any additional binders or conductive additives.More importantly, these properties allow ICCNs to act as both an activematerial and current collector in the EC. The combination of bothfunctions in a single layer leads to a simplified and lightweightarchitecture. Thus, a device can be readily made by sandwiching an ionporous separator [Celgard 3501 (Celgard, Charlotte, N.C.)] between twoICCN electrodes. ICCN ECs are relatively thin with a total thickness ofless than around about 100 mm, making them potentially useful inmicrodevice applications. Other devices can be made by putting ICCNs onporous substrates such as a nitrocellulose membrane or photocopy paperor on conductive aluminum foil, which is often used in commercialdevices. Therefore, ICCN ECs can be readily made into different designs,including stacked and spirally wound structures to target differentapplications.

The ICCN electrodes are fabricated to satisfy the critical features forhigh-performance ECs. First, the relatively large and accessiblespecific surface area of an ICCN (1520 m²/g compared with 1000 to 2000m²/g for a typical activated carbon material) results in a sizeablecharge storage capacity and accounts for the high areal and volumetricstack capacitances observed. Second, the LWL 34 (FIG. 2) that istypically a LightScribe or a LabelFlash laser, causes the simultaneousreduction and partial exfoliation of GO sheets and produces the ICCN 80(FIG. 5). The novel structure of the ICCN 80 is porous, which preventsthe agglomeration of carbon sheets, which has been a major barrier inachieving the full potential of carbon-based ECs. The network structureof the ICCN 80 has open pores, which facilitates electrolyteaccessibility to the electrode surfaces. This offers an opportunity tooptimize ionic diffusion in the electrodes 82, which is crucial forcharging the electrochemical double layers (EDLs), and generates highpower ECs. Moreover, the ICCN 80 possesses excellent electronicconductivity, which is another key factor for achieving high power.Working with these properties, three dimensional composite electrodeshave been successfully used to make batteries with relatively highenergy density and fast charge/discharge rates. Although activatedcarbons can provide high surface area, the difficulty of controllingtheir pore structure and pore size distribution has so far limited theenergy densities and rate capabilities of commercial ECs.

In order to demonstrate the superior performance of ICCN electrodes forelectrochemical energy storage, symmetric ICCN ECs were assembled usingpolyethylene terephthalate (PET) as a thin flexible substrate and anaqueous electrolyte of 1.0 molar (M) phosphoric acid (H₃PO₄). As shownin FIGS. 13A-13F, the ICCN EC performance was analyzed through bothcyclic voltammetry (CV) and galvanostatic charge/discharge (CC)experiments. In comparison with GO, the ICCN EC shows an enhancedelectrochemical performance with a nearly rectangular CV shape at a scanrate of 1000 mV/s, which is indicative of nearly ideal capacitivebehavior (FIG. 13A) even though no metal current collectors, binders, orelectroactive additives were used, as is the case in commercial ECs.Additionally, the ICCN EC is robust enough to be charged and dischargedover a wide range of scan rates (100 to 10,000 mV/s) and still maintainits nearly ideal rectangular CV shape. FIG. 13B shows the nearlytriangular shape of the CC curves obtained at a high current density of10 A/g of ICCN per electrode (abbreviated 10 A/g_(ICCN/electrode)). Thisis indicative of the formation of an efficient EDL and fast iontransport within the ICCN electrodes. In addition, these CC curves showonly a small voltage drop of 0.018 V at the start of the dischargecurve, indicating a device with a low equivalent series resistance(ESR). The specific capacitance from the CC curves was measured over awide range of charge/discharge current densities. Here, the areal andvolumetric capacitances of the stack (this includes the flexiblesubstrate, the current collector, the active material, and theseparator) were calculated and compared with a commercialactivated-carbon EC (AC-EC) tested under the same dynamic conditions.Although the AC-EC shows a slightly higher volumetric capacitance at lowcharge/discharge rates, its capacitance falls off quickly at higherrates, whereas the ICCN EC continues to provide high capacitance evenwhen operated at very high rates (FIG. 13C). In addition, the arealcapacitance of the ICCN EC was calculated to be 3.67 mF/cm² and 4.04mF/cm² in 1.0 M H₂SO₄ at 1 A/g_(ICCN/electrode). The device also shows avery high rate capability while still maintaining a capacitance of morethan 1.84 mF/cm², even when the ICCN EC is operated at an ultrafastcharge/discharge rate of 1000 A/g_(ICCN/electrode). This is comparablewith values reported in the literature for micro-devices and thin filmECs at much lower current charge/discharge rates (0.4 to 2 mF/cm²).These ECs can be efficiently charged/discharged on the 0.1-s time scale.Additionally, the ICCN EC retained around about 96.5% of its initialcapacitance after 10,000 cycles (FIG. 13D).

Electrochemical impedance spectroscopy (EIS) confirmed fast iontransport within the ICCN electrodes. A complex plan plot of theimpedance data of the ICCN EC is shown in FIG. 13E with an expanded viewprovided in the inset. The device displays a pure capacitive behavior,even at high frequencies of up to ˜158 Hz. The series resistance of thedevice is estimated to be ˜16 ohms. This value can be attributed to thecontact resistance of the device with the external circuit that could bereduced by using current collectors. The dependence of the phase angleon the frequency for the ICCN EC, AC-EC, and an aluminum electrolyticcapacitor is shown in FIG. 13F. For frequencies up to 10 Hz, the phaseangle of the ICCN EC is close to −90°, which suggests that the devicefunctionality is close to that of an ideal capacitor. The characteristicfrequency f0 for a phase angle of −45° is 30 Hz for the ICCN EC. Thisfrequency marks the point at which the resistive and capacitiveimpedances are equal. The corresponding time constant t0 (=1/f0) equals33 ms compared with 10 seconds for the conventional AC-EC and 1 ms forthe aluminum electrolytic capacitor. This rapid frequency response ofthe ICCN EC can be accounted for by the large and accessible surfacearea of the ICCN, whose exposed flat sheets enhance the ion transportrate in the device. This is consistent with results reported recentlyfor an EC made from vertically oriented graphene nanosheets growndirectly on metal current collectors and carbon nanotube electrodes madewith an electrophoretic deposition technique.

The future development of multifunctional flexible electronics such asroll-up displays, photovoltaic cells, and even wearable devices presentsnew challenges for designing and fabricating lightweight, flexibleenergy storage devices. Commercially available ECs consist of aseparator sandwiched between two electrodes with liquid electrolyte,which is then either spirally wound and packaged into a cylindricalcontainer or stacked into a button cell. Unfortunately, these devicearchitectures not only suffer from the possible harmful leakage ofelectrolytes, but their design makes it difficult to use them forpractical flexible electronics. Referring to FIG. 14A depicting thestructure of the EC 84, the liquid electrolyte was replaced withpoly(vinyl alcohol) (PVA)-H₃PO₄ polymer gelled electrolyte, which alsoacts as the separator. This electrolyte reduced the device thickness andweight compared with phosphoric acid and simplified the fabricationprocess because it does not require any special packaging materials. Asdemonstrated in FIG. 14B, at any given charge/discharge rate, thespecific capacitance values for the all-solid-state device werecomparable with those obtained with an aqueous electrolyte. Thehigh-rate performance of the EC 84 can be accounted for by the porousstructure of the ICCN electrodes, which can effectively absorb thegelled electrolyte and act as an electrolyte reservoir to facilitate iontransport and minimize the diffusion distance to the interior surfaces.Another key factor is that ICCN electrodes are binder free, thusenabling a reduction in interfacial resistance and enhancing theelectrochemical reaction rate. As illustrated in FIG. 14C, the deviceperformance was completely stable over 4 months of testing. As with theaqueous ICCN EC, the flexible all-solid-state ICCN EC maintains itsexcellent cycling stability: >97% of the initial capacitance wasmaintained even after 10,000 cycles.

In order to evaluate under real conditions the potential ofall-solid-state ICCN ECs, such as the EC 84, for flexible energystorage, a device was placed under constant mechanical stress and itsperformance analyzed. FIG. 14D shows the CV performance of this devicewhen tested under different bending conditions. The bending had almostno effect on the capacitive behavior; it can be bent arbitrarily withoutdegrading performance. Moreover, the stability of the device was testedfor more than 1000 cycles while in the bent state, with only ˜5% changein the device capacitance. This performance durability can be attributedto the high mechanical flexibility of the electrodes along with theinterpenetrating network structure between the ICCN electrodes and thegelled electrolyte. The electrolyte solidifies during the deviceassembly and acts like a glue that holds all the device componentstogether, improving the mechanical integrity and increasing its cyclelife even when tested under extreme bending conditions. Because theincreased cycle life of the present EC has yet to be realized incommercial devices, the present ECs may be ideal for next-generationflexible, portable electronics.

Portable equipment often requires cells packaged either in series, inparallel, or in combinations of the two in order to meet energy andpower requirements. For example, laptop batteries commonly have four3.6-V lithium ion cells connected in series to achieve a voltage of 14.4V, and two in parallel to increase the capacity from 2400 mAh to 4800mAh. Thus, it would be of interest to develop an EC that could exhibitcontrol over the operating voltage and current by using tandem serialand parallel assemblies with minimal energy losses. The performances ofa set of tandem ICCN ECs were evaluated by assembling four devices bothin series and in parallel configurations. Compared with a single EC,which operates at 1.0 V, the tandem series ECs exhibited a 4.0 Vcharge/discharge voltage window. In the parallel assembly, the outputcurrent increased by a factor of 4, and thus the discharge time was fourtimes that of a single device when operated at the same current density.As expected, when the four ECs were combined, two in series and two inparallel, both the output voltage and the runtime (capacitive current)increased by a factor of 2 under the same charge/discharge current. Aswith the single devices, the tandem devices show essentially perfecttriangular CC curves with a miniscule voltage drop, which againindicates excellent capacitive properties with minimal internalresistance. Thus, when used in tandem, the ICCN ECs undergo minimalenergy losses. As a demonstration, a tandem EC's ability to light a redlight-emitting diode (LED) that operates at a minimum voltage of 2 V isshown in the FIGS. 14E and 14F.

An organic electrolyte was also examined, and was discovered to allowthe operation of the devices at higher voltages, thus achieving higherenergy densities. In this case, tetraethylammonium tetrafluoroboratedissolved in acetonitrile was used because this is the most commonorganic electrolyte used in commercial devices. As shown in FIG. 15, theICCN EC again exhibits enhanced performance and rate capabilities whencompared with the commercial AC-EC; this is consistent with the dataacquired in the aqueous and gelled electrolytes. Furthermore, the ICCNEC can be operated over a wider voltage window of 3 V. This ICCN ECoffers a specific capacitance of up to 4.82 mF/cm² (265F/g_(ICCN/electrode)) and retains a capacitance of 2.07 mF/cm² whenoperated at the ultrahigh current density of 1000 A/g_(ICCN/electrode).Recently, room-temperature ionic liquids have been intensively studiedas an attractive alternative to conventional electrolytes for ECsbecause of their high ion density, good thermal stability, andnonvolatility, as well as their wider potential window when comparedwith organic electrolytes. An ICCN EC was fabricated using the ionicliquid 1-ethyl-3-methylimidazoliumtetrafluoroborate (EMIMBF₄) thatexhibited a specific capacitance as high as 5.02 mF/cm² (276F/g_(ICCN/electrode)) and at a wider potential window of 4 V. Aprototype ICCN EC was made and encapsulated in the EMIMBF₄ electrolyte,charged at a constant potential of 3.5 V, and used to energize a red LEDfor ˜24 minutes.

FIG. 16 is a Ragone plot comparing the performance of ICCN ECs withdifferent energy storage devices designed for high powermicroelectronics. FIG. 16 also shows the overall performance of the ICCNECs using various electrolytes. The Ragone plot includes a commercial2.75 V/44 mF AC-EC and a 4 V/500-μAh thin film lithium battery and a 3V/300 μF aluminum electrolytic capacitor, all tested under the samedynamic conditions. The plot shows the volumetric energy density andpower density of the stack for all the devices tested. The ICCN EC canexhibit energy densities of up to 1.36 mWh/cm³, which is a value that isapproximately two times higher than that of the AC-EC. Additionally,ICCN ECs can deliver a power density of around about 20 W/cm³, which is20 times higher than that of the AC-EC and three orders of magnitudehigher than that of the 4 V/500-μAh thin film lithium battery. Althoughthe electrolytic capacitor delivers ultrahigh power, it has an energydensity that is three orders of magnitude lower than the ICCN EC.Because of the simplicity of the device architecture and theavailability of the GO precursor which is already manufactured on theton scale, the ICCN ECs of the present embodiments hold promise forcommercial applications.

Embodiments of the present disclosure also include other types of ECs,such as planer and interdigitated ECs. For example, FIG. 17A shows a setof interdigitated electrodes with dimensions of 6 mm×6 mm, spaced ataround about 500 μm, that are directly patterned onto a thin film of GO.Prior to being patterned, the GO film was deposited on a thin flexiblesubstrate, polyethylene terephthalate (PET), in order to fabricate a setof electrodes that are mechanically flexible. The top arrow points tothe region of the ICCN that makes up the black interdigitatedelectrodes, while the bottom arrow points to the un-reduced GO film.Since the electrodes are directly patterned onto the GO film on aflexible substrate, the need for post-processing such as transferringthe film to a new substrate is unnecessary. Although, if desired, a peeland stick method could be used to selectively lift-off the blackinterdigitated electrodes made of ICCN(s) with e.g. polydimethysiloxane(PDMS) and transfer it onto other types of substrates (FIG. 17B). Thesimplicity of this method allows substantial control over patterndimensions, substrate selectivity and electrical properties of theICCN(s) by controlling laser intensity and thereby the amount ofreduction in each film.

These interdigitated electrodes can, in turn, be used to constructsupercapacitors. FIG. 18A shows an exploded view of amicro-supercapacitor 90 having a first electrode 92 and a secondelectrode 94 that are fabricated from ICCNs made up of a plurality ofexpanded and interconnected carbon layers that are electricallyconductive. It is to be understood that optionally either the firstelectrode 92 or the second electrode 94 can be made of a metal, whilethe remaining one of either the first electrode 92 or the secondelectrode 94 is made of ICCNs. However, the first electrode 92 and thesecond electrode 94 are typically laser scribed from a GO film disposedonto a suitable substrate 96 such as PET or silicon (Si) having aninsulating layer 97 such as a silicon dioxide (SiO₂) layer. A firstconductive strip 98 and a second conductive strip 100 are interfacedwith the first electrode 92 and the second electrode 94 to provideelectrically conductive terminals to couple to external circuitry (notshown). Exemplary external circuitry to be powered by themicro-supercapacitor 90 can be, but is not limited to, integratedcircuits and other electrically powered micro-scale devices. A liner 102that is non-electrically conductive covers the portions of the firstelectrode 92 and the second electrode 94 that are interfaced with thefirst conductive strip 98 and the second conductive strip 100. The liner102 includes a central window through which an electrolyte 104 is placedin contact with the first electrode 92 and the second electrode 94. Apolyimide tape can be used as the liner 102. The electrolyte ispreferably a gel electrolyte such as fumed silica (FS) nano-powder mixedwith an ionic liquid. An exemplary ionic liquid is1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Anothersuitable gel electrolyte is a hydrogel such as poly(vinyl alcohol)(PVA)-H₂SO₄. Other electrolytes are also suitable, but the disclosedelectrolytes provide a voltage window between a maximum charged voltageand a minimum discharged voltage of around about 2.5V.

FIG. 18B depicts the micro-supercapacitor 90 fully assembled. In thisexemplary depiction, the first conductive strip 98 becomes a positiveterminal and the second conductive strip 100 becomes a negativeterminal. It is to be understood that the first conductive strip 98 andthe second conductive strip 100 may be made from an electrical conductorsuch as copper (Cu), aluminum (Al), and/or additional ICCN structures.

FIG. 19A depicts a micro-supercapacitor configuration having a firstelectrode 106 with two extending electrode digits 108A and 108B. Asecond electrode 110 has two extending electrode digits 112A and 112Bthat are interdigitated with the extending electrode digits 108A and108B.

FIG. 19B depicts another micro-supercapacitor configuration having afirst electrode 114 with four extending electrode digits 116A through116D. A second electrode 118 has four extending electrode digits 120Athrough 120D that are interdigitated with the four extending electrodedigits 116A through 116D.

FIG. 19C depicts yet another micro-supercapacitor configuration having afirst electrode 122 with eight extending electrode digits 124A through124H. A second electrode 126 has eight extending electrode digits 128Athrough 128H that are interdigitated with the eight extending electrodedigits 124A through 124H.

FIG. 20 is a table listing exemplary dimensions for themicro-supercapacitors of FIGS. 19A-19C. Referring to both FIG. 20 andFIG. 19A, the extending electrode digits 108A, 108B, 112A, and 112B aredepicted with exemplary individual widths (W) of 1770 μm. The extendingelectrode digits 108A, 108B, 112A, and 112B are depicted with anexemplary length (L) of 4800 μm.

Referring to both FIG. 19B and FIG. 20, the width of the extendingelectrode digits 116A through 116D and the extending electrode digits120A through 120D are depicted with exemplary individual widths of 810μm. Referring to both FIG. 19C and FIG. 20, the extending electrodedigits 124A through 124H and the extending electrode digits 128A through128H are depicted with exemplary individual widths of 330 μm. Theexemplary configurations shown in FIGS. 19A, 19B, and 19C all have anexemplary edge dimension (E) of 200 μm, and an exemplary interspacedimension (I) that separates the first electrodes 106, 114, and 122 fromthe second electrodes 110, 118, and 126 with a serpentine gap. Moreover,the exemplary micro-supercapacitor configurations shown in FIGS. 19A,19B, and 19C each have a total area 40 mm². In regard to themicro-supercapacitor configurations of FIGS. 19A, 19B, and 19C, it is tobe understood that ranges of widths (W) are available for each of thefirst extending electrode digits 108A, 108B, 116A through 116D, and 124Athrough 124H and each of the second extending electrode digits 112A,112B, 120A through 120D, and 128A through 128H. In various exemplaryembodiments, the width (W) of each of the first extending electrodedigits 108A, 108B, 116A through 116D, and 124A through 124H and for eachof the second extending electrode digits 112A, 112B, 120A through 120D,and 128A through 128H are greater than around about 330 μm, or greaterthan around about 810 μm, or greater than around about 1770 μm in width.Moreover, ranges of interspace distance (I) between the first extendingelectrode digits 108A, 108B, 116A through 116D, and 124A through 124Hand each of the second extending electrode digits 112A, 112B, 120Athrough 120D, and 128A through 128H respectively, may be less thanaround about 150 μm, or less than around about 100 μm, or less thanaround about 50 μm. The edge dimension (E) can also have multiple rangesthat are around about the same dimensions as those given for the rangesof width (W). These various dimensions provide various area ranges forthe micro-supercapacitor configurations of FIG. 19A. For example, in oneembodiment, a total geometric area of each of the first electrodes 106,114, and 122 and each of the second electrodes 110, 118 and 126 is lessthan around about 50 mm². In another embodiment, a total geometric areaof each of the first electrodes 106, 114, and 122 and each of the secondelectrodes 110, 118 and 126 is less than around about 40 mm². In yetanother embodiment, a total geometric area of each of the firstelectrodes 106, 114, and 122 and each of the second electrodes 110, 118and 126 is less than around about 30 mm².

It is to be understood that the physical size of the supercapacitors ofthe present disclosure is only limited by the wavelength of light thatexfoliates ICCN patterns into GO. Therefore, supercapacitors producedaccording to the present disclosure range from the macro-scale thatincludes supercapacitors large enough to power electric vehicles andsupply industrial electrical power grids down to nano scalenano-supercapacitors that are useable to power nano sized devices suchas nanoelectromechanical (NEMS) devices.

Between the macro-scale and the nano-scale is a sub-micron scale thatincludes a range of micro-supercapacitors that are usable to powerintegrated circuits. For example, the first electrode and the secondelectrode have dimensions around about a sub-micron range. As such,these micro-supercapacitors can be integrated with integrated circuitrysuch that the integrated circuitry and micro-supercapacitors can befabricated into a single integrated circuit package.

The ICCNs of the present disclosure are also usable to fabricaterelatively large first and second electrodes separated by an electrolytethat provides enough charge storage capacity to power passenger carsized electric vehicles. Moreover, supercapacitors fabricated inaccordance with the present disclosure are also usable to supplyelectrical power to industrial electrical power grids during peak powerdemands. For example, the first electrode and the second electrode of asupercapacitor according to the present disclosure can be sized tosupply peak power to a megawatt capacity electrical power grid.

A process for fabricating the supercapacitors of the present disclosureis schematically illustrated in FIG. 21A. Circuits designed on acomputer can be patterned onto the GO film 72 on the substrate 70 whichis carried by a substrate carrier such as a DVD disc. In the process GOabsorbs high intensity light from a light source such as the laser beam40 and is converted into ICCN(s). By using the precision of a laser suchas the LWL 34, a direct-to-disc labeling drive renders acomputer-designed pattern onto the GO film 72 to produce desired ICCNcircuits. In this way, interdigitated ICCN electrodes 92 and 94 can bereadily scribed on the GO film and transferred to the substrate 96 asshown in FIG. 21B. With its nearly insulating properties, GO serves as agood separator between the positive and negative ICCN interdigitatedelectrodes. These ICCN circuits can thus be directly used as planarmicro-supercapacitors after receiving an electrolyte overcoat, asdepicted in FIG. 21C. Unlike conventional micro-fabrication methods,this direct “writing” technique does not require masks, expensivematerials, post-processing or clean room operations. Furthermore, thetechnique is cost effective and readily scalable. For example, using anexemplary design chosen for this work, 112 micro-supercapacitors 130were produced on a single piece of GO deposited on a flexible DVDdisc-shaped substrate 132 as depicted in FIG. 21D. Interdigitatedelectrodes can be precisely patterned with a lateral spatial resolutionof around about 20 μm using direct-to-disc labeling. This technique isthus appropriate for the fabrication of high-resolutionmicro-supercapacitors taking into account that the interdigitatedelectrodes made with conventional micro-fabrication techniques areusually on the order of around about 100 μm.

The laser scribing process of the present disclosure is associated withsignificant changes in the optical properties, the electrical propertiesand the structure of the film. For example, GO changes from a goldenbrown color to black; a direct impact of the reduction of GO into anICCN. FIG. 22A shows a line drawing of the as-prepared ICCNmicro-supercapacitors 134. In particular, a micro-device 136 having 4interdigitated electrodes, 2 positive and 2 negative; along with anothermicro device having 8 interdigitated electrodes, 4 positive and 4negative; are shown with yet another micro-device 140 with 16interdigitated microelectrodes, 8 positive and 8 negative. FIG. 22B is aline drawing of an optical microscope image showing a well-definedpattern with no short circuits between the microelectrodes. FIG. 22Cshows the expansion of the GO film when treated with the laser, thusenabling full access to the electrode surface that is essential forcharging the electrodes. Analysis of the cross-section of themicro-device reveals a thickness of 7.6 μm. For comparison, I-V testswere carried out for both GO and an ICCN as shown in FIGS. 22D and 22E,respectively. The GO film exhibits nonlinear and slightly asymmetricbehavior with a differential conductivity value ranging from aroundabout 8.07×10⁻⁴ through 5.42×10⁻³ S/m depending on the gate voltage.Reducing GO within the direct-to-disc labeling laser results in a linearI-V curve associated with a significant increase in the filmconductivity to around about 2.35×10³ S/m as calculated for the ICCN asdepicted in FIG. 22F. Because of its high electrical conductivity andexceptionally high surface area of over 1500 m²/g, the ICCN can serve asboth the electrode material and current collector. This simplifies thefabrication process and results in cost-effective micro-supercapacitors.

In order to understand the role of the micro-scale architecture of thedevice on its electrochemical properties, different configurations weredesigned and tested. Micro-supercapacitors with 4 (MSC4), 8 (MSC8), and16 (MSC16) interdigitated microelectrodes were constructed and theirelectrochemical performance at 1,000, 5,000 and 10,000 mV/s tested, asshown in FIGS. 23A-23C. A hydrogel-polymer electrolyte, PVA-H₂SO₄, wasused to fabricate the all-solid-state micro-supercapacitors. Asandwich-type ICCN supercapacitor was also tested for comparison.

The CV profiles are all rectangular in shape, confirming the formationof an efficient electrochemical double layer (EDL) capacitor and fastcharge propagation within the electrodes. Even at an ultrafast scan rateof 10,000 mV/s, the CV remains rectangular in shape indicating the highpower capability of this micro-supercapacitor. Volumetric and arealcapacitances give a more accurate picture of the true performance of asupercapacitor compared with gravimetric values. This is even morerelevant in the case of micro-devices since the mass of the activematerial is very small. Therefore, calculations of the specificcapacitance of the micro-devices have been made based on the volume ofthe stack. This includes the combined volume of the active material,current collector and the gap between the electrodes. The stackcapacitances of the different micro-supercapacitors as a function of thescan rate are shown in FIG. 23D. Interestingly, the micro-devices showhigher capacitance when using the interdigitated structure as opposed tothe sandwich structure. Furthermore, the more interdigitated electrodesper unit area, the more power and energy can be extracted from themicro-devices. This can be explained by the unique porous networkstructure of the ICCN electrodes that helps minimize the pathway for iondiffusion from the electrolyte to the electrode material. Moreover, themicro-scale architecture of the devices results in a significantreduction of the mean ionic diffusion pathway between twomicroelectrodes. This effect becomes even more pronounced whenincreasing the number of interdigitated electrodes per unit area. Thisallows for maximizing the available electrochemical surface area andresults in the increased capacitance and the fast charge/discharge ratesobserved with the micro-devices.

These conclusions are confirmed by the galvanostatic charge/discharge(CC) curves depicted in FIG. 23E. Note that all the micro-devices,regardless of whether they possess 4, 8 or 16 interdigitated electrodes,show nearly ideal triangular CC curves obtained at an ultrahigh currentdensity of around about 1.684×10⁴ mA/cm³. The voltage drop at thebeginning of each discharge curve, known as the iR drop, is a measure ofthe overall resistance of the device and since its value is proportionalto the discharge current, the small iR drop shown in FIG. 23E at a highdischarge current indicates a very low resistance for allmicro-supercapacitors tested.

The iR drop gradually decreases from ICCN-MSC(4) through ICCN-MSC(16),thus confirming the increase in power density of the micro-devices withan increasing number of interdigitated electrodes per unit area. FIG.23F shows the volumetric capacitance of the stack as a function of thecurrent density for the ICCN micro-supercapacitor for both theinterdigitated and sandwich structures. For comparison, the data for acommercial activated carbon supercapacitor obtained under the samedynamic conditions is also shown. Not only does the activated carbonsupercapacitor exhibit lower capacitance, but its performance falls offvery quickly at higher charge/discharge rates because of the limiteddiffusion of ions in the inner porous network of the activated carbon.The surface of the ICCN, on the other hand, is highly accessible to theelectrolyte with very little impediment to ion transport, thus providinghigh capacitance even when operated at ultrahigh charge/discharge rates.For example, ICCN-MSC(16) exhibits a stack capacitance of around about3.05 F/cm³ at 16.8 mA/cm³ and maintains 60% of this value when operatedat an ultrahigh current density of 1.84×10⁴ mA/cm³ (FIG. 23F). This isequivalent to the operation of the device at around about 1100A/g_(ICCN/electrode) which is around about three orders of magnitudehigher than the normal discharge current densities used for testingtraditional supercapacitors. This corresponds to an areal capacitancethat varies only slightly from around about 2.32 mF/cm² at 16.8 mA/cm³to 1.35 mF/cm² at 1.84×10⁴ mA/cm³. Moreover, in traditionalsupercapacitors made of activated carbon, most of the surface arearesides in the micropores of the carbon; as such, this is unlikely tocontribute significantly to the charge storage, especially at a highrate. This results in a poor frequency response, with the energy storedin these carbon electrode materials released only at slow rate. On theother hand, the ICCN, with its sheet-like structure, possesses a largeopen surface area that is readily accessible to an electrolyte with asmall diffusion barrier. Thus, the ICCN has the potential for makingsupercapacitors with power densities that surpass any other form ofactivated carbon. The superior frequency response achieved with ICCNmicro-devices is due to the excellent electrolyte access to the surfacesof carbon sheets through its interconnected pores. The micro-scaledesign of ICCN devices improves the rate capability through thereduction of the ion transport pathways. In addition, ICCN forms ahighly conductive network, thus reducing the internal resistance ofmicroelectrodes that make up micro-supercapacitors.

FIG. 23G is a graph of a complex plane plot of the impedance of anICCN-MSG(16) with a magnification of a high frequency region shown in aninset. FIG. 23H is a graph of impedance phase angle versus frequency foran ICCN-MSG(16) compared to commercial AC-SC and aluminum electrolyticcapacitors. FIG. 23I is a graph showing a relatively high amount ofcapacitance retention over at least 10,000 charge and discharge cycles.In particular, the graph of FIG. 23I shows only a loss of around about4% of initial capacitance over 10,000 charge and discharge cycles.

Flexible electronics have recently attracted much attention because oftheir potential in providing cost-efficient solutions to large-areaapplications such as roll-up displays and TVs, e-paper, smart sensors,transparent RFIDs and even wearable electronics. However, thefabrication of micro-supercapacitors on flexible substrates usingcurrent micro-fabrication techniques does not appear to be feasible.Attempts to fabricate micro-supercapacitors on flexible substrates usinga number of printing and electrochemical techniques have also beenreported. However, none of these configurations has been shown to besuitable for flexible energy-storage devices. In fact, the performancedurability of these devices has not been examined under any strainconditions such as bending or twisting. ICCN micro-supercapacitors suchas micro-supercapacitor 90 are highly flexible and can be bent andtwisted without affecting the structural integrity of the device, FIG.24A. The durability of ICCN micro-supercapacitors for flexible energystorage has been demonstrated by tests of their electrochemicalperformance under constant strain. FIG. 24B shows the CV performance ofthe micro-supercapacitor with different bending and twisting conditionsat 1,000 mV/s. The micro-supercapacitor shows exceptionalelectrochemical stability regardless of the degree of bending ortwisting, indicating excellent mechanical stability. The flexibilityendurance of the device was tested while keeping the device under thebent or twisted state, as depicted in FIG. 24C. Remarkably, thecapacitance was reversibly maintained with 97% retention of the initialcapacitance after 2,000 cycles. This superior performance makes ICCN-MSCpromising for flexible micro-electronics.

In general, the total energy that can be stored in a singlesupercapacitor is too low for most practical applications. Thus,depending on the application, supercapacitors need to be connectedtogether in series and/or parallel combinations, just as batteries are,to form a “bank” with a specific voltage and capacitance rating. Theadaptability of ICCN micro-supercapacitors for serial/parallelcombinations is demonstrated by connecting four devices together both inseries and in parallel configurations, as depicted in FIGS. 24D-24F. Thetandem ICCN micro-supercapacitors exhibit a very good control over theoperating voltage window and capacity, thus enabling them to beconsidered for practical applications. Like the individualmicro-supercapacitors, the tandem devices exhibit essentially idealtriangular CC curves with a minute voltage drop, which again indicatesexcellent capacitive properties with minimal internal resistance. It isworth noting that this exceptional performance has been achieved withoutusing a voltage balance, which is often needed with series connectionsto prevent any cell from going into over-voltage.

Previous research attempts to design supercapacitors in theall-solid-state form have focused mainly on using aqueoushydrogel-polymer electrolytes. Unfortunately, the operating voltagerange of these devices barely exceeds 1 V, making them non-functionalfor many applications. Unlike water-based electrolytes, ionic liquids(IL) provide an attractive alternative to these conventionalelectrolytes owing to their wide electrochemical window and high ionicconductivity as well as good thermal stability and non-volatility. Theseinteresting properties of ILs can be hybridized with another solidcomponent (e.g. polymer, silica, etc.) to form gel-like electrolytescalled ionogels.

The combination of a solid matrix with ILs preserves the main propertiesof ILs, while allowing easy shaping of the device without having theintrinsic leakage problems of liquid electrolytes that limit theirflexible operation. Although promising, the integration of ionogels intoall-solid-state micro-supercapacitors has not yet been demonstrated.Here, fumed silica (FS) nano-powder was mixed together with the ionicliquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide toform a clear viscous (FS-IL) ionogel 142, as depicted in FIG. 25A.

In an exemplary embodiment, the ionogel is prepared by mixing together afumed silica nano-powder having an average particle size 7 nm with theionic liquid (1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide ([BMIM][NTf2])) (0.03 g FS/1.0 g([BMIM][NTf2]). This mixture is then stirred under an Argon atmospherefor 5 hours to produce a clear viscous ionogel (FS-IL). The ionogel isthen usable as an electrolyte for the fabrication of all-solid-statemicro-supercapacitors that are capable of providing 2.5 V compared with1 V for traditional hydrogel-polymer electrolytes. Resultingmicro-supercapacitors thus have a potential for increased energy densityby at least one order of magnitude. The ionogel is integrated into anall-solid-state micro-supercapacitor. Interestingly, the all-solid-statemicro-supercapacitor demonstrates ultrahigh charge/discharge ratescomparable to those with PVA-H₂SO₄ hydrogel electrolyte. However, as aresult of the ionogel electrolyte, the all-solid-statemicro-supercapacitor can be operated at a larger potential window of 2.5V.

The almost ideal CV profiles and triangular CC curves at ultrafastcharge/discharge rates verify good EDLC behavior. The ICCN-MSC(16)achieved a stack capacitance of 2.35 F/cm³ at a current density of 16.8mA/cm³. When operated at an ultrafast charge/discharge current densityof 1.84×10⁴ mA/cm³, the capacitance of the device drops only slightly to1.40 F/cm³. Since the energy density increases with the square of theoperating potential window, the micro-supercapacitor employing a FS-ILionogel promises an order of magnitude higher energy density.Furthermore, the high thermal stability of ionic liquids eliminates thefire hazards associated with commercial supercapacitors. Finally, themicro-supercapacitor shows excellent cycling stability; the capacitanceremains unchanged after more than 30,000 charge/discharge cycles.

Current trends for developing miniaturized electronic devices placeemphasis on achieving performance levels generally associated withintegrated circuits. FIG. 25B depicts an exemplary on-chipmicro-supercapacitor 144 that can be integrated with MEMS devices andCMOS in a single chip using the direct-to-disc labeling technique. Astructure made up of a silicon (Si) substrate and a silicon dioxide(SiO₂) insulating layer for the on-chip micro-supercapacitor 144 isschematically illustrated in FIG. 25B; with the ionogel 142 used as theelectrolyte. Other devices 146 similar to the micro-supercapacitor 144were fabricated using the same process described earlier except for theplastic substrate which has been replaced with an oxidized silicon wafer148, as depicted in FIG. 25C. FIGS. 26D-26E show that the device revealssuperior electrochemical performance with ultrahigh power, comparable tothat demonstrated on the flexible substrate. This technique may thuspresent a low-cost and scalable solution for on-chip self-poweredsystems.

Charged supercapacitors, like charged batteries, are in a state of highfree energy relative to that of the discharged state, so there is athermodynamic driving force for them to self-discharge. Theself-discharge behavior of supercapacitors is a matter of majorpractical significance in their operation and the types of function theymay be required to fulfill. During self-discharge, a small amount ofleakage current will cause the voltage decay of a charged supercapacitorover time. The leakage current can be measured by applying a rated DCvoltage to the supercapacitor and measuring the current required tomaintain that voltage. Typically, this is done using the voltage atwhich the supercapacitor is operated, Vmax. The results are presented inFIG. 26A which also include the data for two commercially availablesupercapacitors, all tested under the same dynamic conditions. Theresults show that the ICCN micro-supercapacitor exhibits an ultra-smallleakage current of less than around about 150 nA after 12 hours comparedto less than around about 5 μA for both of the commercialsupercapacitors. With its low leakage current, ICCNmicro-supercapacitors could be integrated with energy harvesters tocreate efficient self-powered systems.

The self-discharge curves obtained immediately after pre-charging toVmax in the previous test are shown in FIG. 26B. Basically, the voltagedifference between the two terminals of the supercapacitor is recordedon open circuit as a function of time. Normally, most supercapacitorsare operated in the range of Vmax to approximately ½Vmax. Thus the timerequired for the voltage across the supercapacitor to change from Vmaxto ½Vmax was measured for all of the tested supercapacitors. The resultsshow that the ICCN micro-supercapacitor self-discharges in 13 hours, avalue comparable to those of commercial supercapacitors withself-discharge rates of 8 hours and 21 hours. This fine performance forthe ICCN micro-supercapacitors shows promise for practical applications.

FIG. 27 shows a Ragone plot comparing the performance of ICCNmicro-supercapacitors with different energy storage devices designed forhigh-power microelectronics. The Ragone plot shows the volumetric energydensity and power density of the stack for all the devices tested. TheRagone plot reveals a significant increase in supercapacitor performancewhen scaling down the electrode dimensions to the micro-scale. Forexample, the interdigitated micro-supercapacitors deliver more energyand power than their sandwich counterparts both in the hydrogel-polymerand ionogel electrolytes. Remarkably, compared with the ACsupercapacitor, the ICCN micro-device exhibits three times more energyand around about 200 times more power. Furthermore, the ICCNmicro-supercapacitors demonstrate power densities comparable to those ofthe aluminum electrolytic capacitor, while providing more than threeorders of magnitude higher energy density. Although Li-ion batteries canprovide high energy density, they have limited power performance that is4 orders of magnitude lower than the ICCN-MSC. This superior energy andpower performance of the ICCN micro-supercapacitors should enable themto compete with micro-batteries and electrolytic capacitors in a varietyof applications. Further miniaturization of the width of themicro-electrodes and the space between them would reduce the ionicdiffusion pathway, thus leading to micro-supercapacitors with evenhigher power density.

The single-step fabrication technique described here obviates the needfor time-consuming and labor-intensive lithography, while enhancing theyield of the process and the functionality of the micro-devicesproduced. Remarkably, this technique allows for the fabrication ofmicro-devices without the use of organic binders, conductive additivesor polymer separators that are often needed in commercialsupercapacitors, thus leading to improved performance because of theease with which ions can access the active material. The combination ofthe microscale design of the device with the ICCN whose surface is fullyaccessible to electrolyte ions is responsible for the high power/energyperformance of the ICCN micro-supercapacitors. They combine the powerdensity of electrolytic capacitors with the energy density ofmicro-batteries that could have a significant impact on high-powermicroelectronics. These findings also provide a solution to microscaleenergy storage in numerous areas where electrolytic capacitors cannotprovide sufficient volumetric energy density.

Furthermore, ICCN micro-supercapacitors show excellent cyclingstability. This is relatively important when compared withmicro-batteries whose finite life-time could present significantproblems when embedded in permanent structures such as biomedicalimplants, active radio frequency identification (RFID) tags and embeddedmicro-sensors where no maintenance or replacement is possible. Sincethese micro-supercapacitors can be directly integrated on-chip, they mayhelp to better extract the energy from solar, mechanical, and thermalsources and thus enable more efficient self-powered systems. They couldalso be fabricated on the backside of solar cells in both portabledevices and rooftop installations, to store power generated during theday for use after sundown and thus may help to provide electricityaround the clock where connection to the grid is not possible. Otherapplications may arise which take advantage of the flexible nature ofthe substrates, such as electronics embedded into clothing, large-areaflexible displays, and roll-up portable displays.

Note that the electrodes made of ICCNs are fabricated on flexible PETsubstrates covered with GO which, when laser reduced, serves as both theelectrode and the current collector, thus making this particularelectrode not only lightweight and flexible, but also inexpensive. Inaddition, the low oxygen content in ICCNs (˜3.5%) as shown through XPSanalysis is quite advantageous to the electrochemical activity seenhere, since a higher oxygen content at the edge plane sites have beenshown to limit and slow down the electron transfer of theferri-/ferrocyanide redox couple. As such, embodiments of the presentdisclosure provide methodologies for making highly electroactiveelectrodes for potential applications in vapor sensing, biosensing,electrocatalysis and energy storage.

The present disclosure relates to a facile, solid-state andenvironmentally safe method for generating, patterning, and electronictuning of graphite-based materials at a low cost. ICCNs are shown to besuccessfully produced and selectively patterned from the direct laserirradiation of GO films under ambient conditions. Circuits and complexdesigns are directly patterned on various flexible substrates withoutmasks, templates, post-processing, transferring techniques, or metalcatalysts. In addition, by varying the laser intensity and laserirradiation treatments, the electrical properties of ICCNs are preciselytuned over at least five orders of magnitude, a feature that has provendifficult with other methods. This new mode of generating ICCNs providesa new venue for manufacturing all organic based devices such as gassensors, and other electronics. The relatively inexpensive method forgenerating ICCNs on thin flexible organic substrates makes it arelatively ideal heterogeneous scaffold for the selective growth ofmetal nanoparticles. Moreover, the selective growth of metalnanoparticles has the potential in electrocatalysing methanol fuelcells. Further still, films made of ICCNs show exceptionalelectrochemical activity that surpasses other carbon-based electrodes inthe electron charge transfer of ferro-/ferricyanide redox couple. Thesimultaneous reduction and patterning of GO through the use of aninexpensive laser is a new technique, which offers significantversatility for the fabrication of electronic devices, all organicdevices, asymmetric films, microfluidic devices, integrated dielectriclayers, batteries, gas sensor, and electronic circuitry.

In contrast to other lithography techniques, this process uses alow-cost infrared laser in an unmodified, commercially available CD/DVDoptical disc drive with LightScribe technology to pattern complex imageson GO and has the additional benefit to simultaneously produce the laserconverted corrugated carbon network. A LightScribe technology laser istypically operated with a 780 nm wavelength at a power output within arange of around 5 mW to around 350 mW. However, it is to be understoodthat as long as the carbon-based oxide absorbs within the spectrum ofthe laser's emission, the process is achievable at any wavelength at agiven power output. This method is a simple, single step, low cost, andmaskless solid-state approach to generating ICCNs that can be carriedout without the necessity of any post-processing treatment on a varietyof thin films. Unlike other reduction methods for generatinggraphite-based materials, this method is a non-chemical route and arelatively simple and environmentally safe process, which is not limitedby chemical reducing agents.

The technique described herein is inexpensive, does not require bulkyequipment, displays direct control over film conductivity and imagepatterning, can be used as a single step for fabricating flexibleelectronic devices, all without the necessity for sophisticatedalignment or producing expensive masks. Additionally, due to theconductive nature of the materials used, it is possible to control theresulting conductivity by simply patterning at different laserintensities and power, a property that has yet to have been shown byother methods. Working circuit boards, electrodes, capacitors, and/orconducting wires are precisely patterned via a computerized program. Thetechnique allows control over a variety of parameters, and thereforeprovides a venue for simplifying device fabrication and has thepotential to be scaled, unlike other techniques that are limited by costor equipment. This method is applicable to any photothermically activematerial, which includes but is not limited to GO, conducting polymers,and other photothermically active compounds such as carbon nanotubes.

As described above, a method has been presented for producinggraphite-based materials that is not only facile, inexpensive andversatile, but is a one-step environmentally safe process for reducingand patterning graphite films in the solid state. A simple low energy,inexpensive infrared laser is used as a powerful tool for the effectivereduction, subsequent expansion and exfoliation and fine patterning ofGO. Aside from the ability to directly pattern and effectively producelarge areas of highly reduced laser converted graphite films, thismethod is applicable to a variety of other thin substrates and has thepotential to simplify the manufacturing process of devices made entirelyfrom organic materials. A flexible all organic gas sensor has beenfabricated directly by laser patterning of GO deposited on thin flexiblePET. An ICCN is also shown to be an effective scaffold for thesuccessful growth and size control of Pt nanoparticles via a simpleelectrochemical process. Finally, a flexible electrode made of ICCN wasfabricated, which displays a textbook-like reversibility with animpressive increase of ˜238% in electrochemical activity when comparedto graphite towards the electron transfer between theferri-/ferrocyanide redox couple. This exemplary process has thepotential to effectively improve applications that would benefit fromthe high electrochemical activity demonstrated here including batteries,sensors and electrocatalysis.

Those skilled in the art will recognize improvements and modificationsto the embodiments of the present disclosure. All such improvements andmodifications are considered within the scope of the concepts disclosedherein and the claims that follow.

1. A micro-supercapacitor comprising: a. a first electrode; b. a secondelectrode; c. a first conductor interfaced with the first electrode; d.a second conductor interfaced with the second electrode; e. anon-conductive liner contacting at least a portion of the firstconductor and the second conductor; and f. an electrolyte in contactwith the first electrode and the second electrode, wherein at least oneof the first electrode or the second electrode comprises aninterconnected corrugated carbon-based network (ICCN) having a pluralityof expanded and interconnected carbon layers.
 2. Themicro-supercapacitor of claim 1, wherein the non-conductive linercontacting at least the portion of the first conductor and the secondconductor provides an electrical terminal configured to be coupled toexternal circuitry.
 3. The micro-supercapacitor of claim 1, wherein thenon-conductive liner seals the electrolyte within themicro-supercapacitor device.
 4. The micro-supercapacitor of claim 1,wherein the non-conductive liner contacts at least a portion of thefirst electrode or the second electrode.
 5. The micro-supercapacitor ofclaim 1, wherein the non-conductive liner comprises a central window. 6.The micro-supercapacitor of claim 1, wherein the electrolyte comprises agel electrolyte.
 7. The micro-supercapacitor of claim 1, wherein theelectrolyte comprises fumed silica (FS) nano-powder mixed with an ionicliquid.
 8. The micro-supercapacitor of claim 1, wherein the electrolytecomprises 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.9. The micro-supercapacitor of claim 1, wherein the electrolytecomprises a hydrogel electrolyte.
 10. The micro-supercapacitor of claim1, wherein the electrolyte comprises poly(vinyl alcohol) (PVA)-H₂SO₄.11. The micro-supercapacitor of claim 10, wherein themicro-supercapacitor is a solid state micro-supercapacitor.
 12. Themicro-supercapacitor of claim 1, wherein the first conductive strip orthe second conductive strip comprises copper (Cu), aluminum (Al), or anICCN structure.
 13. The micro-supercapacitor of claim 1, wherein thefirst electrode comprises a first plurality of electrode digits, whereinthe second electrode comprises a second plurality of electrode digits,and wherein the first plurality of electrode digits of the firstelectrode and the second plurality of electrode digits of the secondelectrode are woven in an interdigitating pattern.
 14. Themicro-supercapacitor of claim 13, further comprising an interspacedistance between the interdigitating pattern of the first electrode andthe second electrode of about 150 micrometers.
 15. Themicro-supercapacitor of claim 1, wherein the first electrode or thesecond electrode comprises 4, 8, or 16 electrode digits.
 16. Themicro-supercapacitor of claim 1 wherein the first electrode or thesecond electrode comprises widths from about 330 micrometer to about1770 micrometers.
 17. The micro-supercapacitor of claim 1, furthercomprising a total area of less than about 50 square millimeters, 40square millimeters, or 30 square millimeters.
 18. Themicro-supercapacitor of claim 1, wherein the first electrode or thesecond electrode comprises an edge dimension of from about 220micrometers to about 1770 micrometers.
 19. The micro-supercapacitor ofclaim 1, wherein the micro-supercapacitor is comprised within anintegrated circuit package or a nanoelectromechanical device.
 20. Themicro-supercapacitor of claim 1, wherein the micro-supercapacitorexhibits a nearly triangular CC curve when operated at a first currentdensity of about 1.684×10⁴ mA/cm³, wherein the micro-supercapacitorexhibits a stack capacitance of around about 3.05 F/cm³ at 16.8 mA/cm³,and wherein the micro-supercapacitor maintains about 60% of the stackcapacitance when operated at a second current density of about 1.84×10⁴mA/cm³.